Which is Better: A Low-Fat or Low-Carbohydrate Diet | Results from the DIETFITS Study

The Diet Intervention Examining The Factors Interacting with Treatment Success (DIETFITS) study examined whether genotypes, differences in insulin secretion, or both, predispose individuals to differential success in 12-month weight change while on a low-fat diet vs a low-carbohydrate diet.

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The DIETFITS study was based out of Stanford University and was a single-site, parallel-group, trail that randomized individuals to a healthy low-fat diet or a healthy low-carbohydrate diet for 12 months.

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Individuals were instructed to reduce their intake of total fat or carbohydrates to 20 g/d during the first 8 weeks. For example, in the low-fat group, the amount of edible oils, fatty meats, whole-fat dairy, and nuts were reduced, whereas in the low-carbohydrate group the amount of cereals, grains, rice, starchy vegetables, and legumes were reduced. Carbohydrates or fats were then introduced back slowly to their diets in increments of 5 to 15 g/day per week until they reached the lowest level of intake they believed could be maintained indefinitely. No explicit instructions for energy (kilocalories) restriction were given.
Both diet groups were instructed to (1) maximize vegetable intake; (2) minimize intake of added sugars, refined flours, and trans fats; and (3) focus on whole foods that were minimally processed, nutrient dense, and prepared at home whenever possible.

Results

A total of 609 individuals completed the study. Among participants in the healthy low-fat diet group, 130 (42.6%) had the low-fat genotype and 83 (27.2%) had the low-carbohydrate genotype, whereas in the healthy low-carbohydrate group, 114 (37.5%) had the low-fat genotype and 97 (31.9%) had the low- carbohydrate genotype.

In the healthy low-fat diet vs the healthy low-carbohydrate diet, the mean 12-month macronutrient distributions were 48% vs 30% for carbohydrates, 29% vs 45% for fat, and 21% vs 23% for protein, respectively.

Primary Outcome: Change in Weight at 12 months

The mean 12-month weight change was −5.3 kg (95% CI, −5.9 kg to −4.7 kg) for the healthy low-fat diet group and −6.0 kg (95% CI, −6.6 kg to −5.4 kg) for the healthy low- carbohydrate diet group, which was not statistically different.

In addition, there was no significant difference in weight change among participants matched vs mismatched to their diet assignment based on their genotype pattern. There was also no significant differences in body mass index, body fat percentage, and waist circumference between the two groups.

Secondary Outcomes: Changes in Cholesterol

At 12 months, both diets improved lipid profiles and lowered blood pressure, insulin, and glucose levels.
The 12-month changes in low-density lipoprotein (LDL) cholesterol concentrations significantly favored a healthy low-fat diet. Whereas, High-density lipoprotein (HDL) cholesterol concentrations increased significantly more and concentrations of triglycerides decreased significantly more for the healthy low-carbohydrate diet group.

In Conclusion

The DIETFITS Study showed that there is no significant difference in weight change between a healthy low-fat diet vs a healthy low-carbohydrate diet, even after adjusting for genotype pattern and baseline insulin secretion.

Citation: Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial

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